This invention relates to processing lignite and, more particularly to, a process for forming high strength pellets from lignite having an increased heating value.
Lignite deposits in North Dakota and Montana, totaling more than 200 billion tons, constitute one of the largest energy sources in the United States. Much of this lignite is located close to the earth's surface and can be conveniently and inexpensively mined. However, it has a high moisture content (30-40%) and relatively low calorific or heating value (about 6,500 Btu/lb). Consequently, the market for lignite extends only a few hundred miles from the mining sites because of the transportation cost per delivered Btu exceeds that of higher ranked fuel, such as bituminous coal and higher grades of sub-bituminous coal.
Also, untreated lignite has very poor handling, storage and transportation properties. It tends to break down from the mechanical handling and exposure to air, readily absorbs and releases moisture, is subject to spontaneous combustion during storage because of its high reactivity with oxygen, and breaks down when subjected to freeze and thaw cycles during outdoor storage.
Attempts have been made to agglomerate lignite into briquettes and pellets having sufficient strength for transportation and handling and a moisture content low enough to increase the heating value to a more competitive level, for example, up to 9,500 Btu/lb or more. In one type of agglomeration, lignite briquettes are produced by hot or cold briquetting without binders. In another type of agglomeration technique, inorganic and organic binders, including spent sulfite liquor, derivatives of petroleum and coal distillates, and synthetic resins have been used for agglomerating lignite fines.
While agglomerates having acceptable mechanical strengths and more economical heating values can be produced by some of these techniques, processing costs increases the total cost of the product to a point where it is no longer competitive with other fossil fuels.
Bituminous type binders, particularly asphalt, pitch and tar, can be advantageous because of their low cost and availability, they contain low amounts of objectionable constituents, such as ash, and alkali, they are capable of imparting water-proofing properties, and they add Btu value to the lignite agglomerates. When used as a binder for agglomerating lignite, asphalt usually is sprayed in molten form onto finely-divided particles of lignite. In order to obtain acceptable binding, the normally wet lignite, as received from the mine, must be dried prior to the application of the asphalt. The molten asphalt tends to solidify quite quickly which complicates application and subsequent agglomeration, with a resultant increase in the cost of the final product. Molten asphalt also tends to generate noxious vapors.